The vertebrate immune system protects the body against undesirable foreign matter that enters the body, such as infecting pathogens (e.g., bacteria, viruses, fungi, and parasites) and their by-products. One manner by which this takes place involves the adaptive immune system, through which the body recognizes foreign antigens and generates specific immune responses against them. The induction of adaptive immunity takes time (e.g., 2–3 days post infection), and thus could leave the body vulnerable to the adverse effects of early infection, if it were not for the action of another division of the immune system, the innate immune system.
The innate immune system provides the body with a first line defense against invading pathogens. In an innate immune response, an invading pathogen is recognized by a germline-encoded receptor, the activation of which initiates a signaling cascade that leads to the induction of cytokine expression. Innate immune system receptors have broad specificity, recognizing molecular structures that are highly conserved among different pathogens. These receptors are known as Toll-like receptors (TLRs), due to their homology with receptors that were first identified and named in Drosophila, and are present in cells such as macrophages, dendritic cells, and epithelial cells.
There are at least ten different TLRs in mammals, and ligands and corresponding signaling cascades have been identified for some of these receptors. For example, TLR2 is activated by the lipoprotein of bacteria (e.g., E. coli.), TLR3 is activated by double-stranded RNA, TLR4 is activated by lipopolysaccharide (i.e., LPS or endotoxin) of Gram-negative bacteria (e.g., Salmonella and E. coli O157:H7), TLR5 is activated by flagellin of motile bacteria (e.g., Listeria), and TLR9 is activated by unmethylated CpG sequences of pathogen DNA. The stimulation of each of these receptors leads to activation of the transcription factor NF-κB, and other signaling molecules that are involved in regulating the expression of cytokine genes, including those encoding tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), and certain chemokines.